What does Genesis 21:34 mean?

Earlier in Genesis, God had promised to make Abraham a great nation and to give to Abraham's descendants all of the land of Canaan (Genesis 12:1–3). For much of Abraham's life, that seemed very unlikely, at least from a human perspective. Now, though, it had at least begun. Isaac had been born, a miraculous birth to a married couple decades beyond normal child-rearing ages (Genesis 21:1–3). Abraham now formally owned a well in Canaan, and had come to a covenant agreement with the king of Gerar, Abimelech (Genesis 21:22–31).

Abraham continued to live in this region of Beersheba for "many days." This area is called "the land of the Philistines." This may be because the people who lived there in Abraham's day were the ancestors of those later to be called Philistines. It is also possible that the writer is referring to this region by the name of those who would later occupy it.